page 26

• How do we find the cosine of the difference of two angles and the cosine of the sum of two angles? (These formulas will be on the Regents Reference sheet)

By the end of this lesson, you will be able to...

• apply the formulas for cos(A - B) and cos(A + B) to find the exact value involving angles measured in radians and degrees
• state the sum and difference formulas in words

Answer these two questions in your notebooks:

1. What is the Cosine sum formula?
2. What is the cosine difference formula?

Video 1


Video 2

• Classwork: Now try these problems. Remember to use the exact measurement. Check your answers on your calculator.

1. Find the exact value of cos 105° (Ans: (sqrt 2 - sqrt 6)/4)
2. Find the exact value of cos 15° (Ans: (sqrt 6 + sqrt 2)/4)

• Homework: Complete those that are multiples of three

Cosine_Sum.JPG

Cosine_Difference.JPG

• Answers:

A-Cosine_Sum.JPG

A-Cosine_Difference.JPG

• Journal entry: Explain why we cannot use the Distributive law to evaluate cos(A + B) as cosA + cosB.